The invention pertains to methods and circuits for adaptive equalization of signals transmitted over a link (e.g., over a cable).
The term xe2x80x9ctransmitterxe2x80x9d is used herein in a broad sense to denote any device capable of transmitting data over a link (e.g., a serial link), and optionally also capable of performing additional functions which can include encoding and/or encrypting the data to be transmitted. The term xe2x80x9creceiverxe2x80x9d is used herein in a broad sense to denote any device capable of receiving data that has been transmitted over a link (e.g., a serial link), and optionally also capable of performing additional functions, which can include decoding and/or decryption of the received data, and other operations related to decoding, reception, or decryption of the received data. For example, the term receiver can denote a transceiver that performs the functions of a transmitter as well as the functions of a receiver.
During high-speed serial data transmission over a link (e.g., a cable and/or PCB traces and/or connectors), the link itself introduces losses and dispersion which reduce the signal quality at the receiver end. As the frequency of the transmitted signal and/or the transmission distance increases, the distortion due to frequency dependent delay and attenuation increases, increasing the chance of false detection of signals received and in some cases making the eye at the receiver almost unusable.
Transmission of signals indicative of data (e.g., video or audio data) to a receiver over a serial link degrades the data, for example by introducing time delay error (sometimes referred to as jitter) to the data. In effect, a link applies a filter to the signals during propagation over the link. The filter (sometimes referred to as a xe2x80x9ccable filter,xe2x80x9d although the link may consist of or include PCB traces) can cause inter-symbol interference (ISI).
Equalization is the application of an inverted version of a cable filter to signals received after propagation over a link. The function of an equalization filter (sometimes referred to as an xe2x80x9cequalizerxe2x80x9d) is to compensate for, and preferably cancel, the cable filter.
Adaptive equalization has been used to restore signal integrity by compensating for the frequency dependent attenuation that occurs in high speed serial data transmission. Various methods have been developed for adaptive equalization.
FIG. 1 is a block diagram of a conventional continuous-time adaptive cable equalizer, of a type described in J. N. Babanezhad, xe2x80x9cA 3.3-V Analog Adaptive Line-Equalizer for Fast Ethernet Data Connection,xe2x80x9d IEEE CICC, pp. 343-346, 1998, and in G. P. Hartman et al., xe2x80x9cContinuous-Time Adaptive-Analog Coaxial Cable Equalizer in 0.5 xcexcm CMOS,xe2x80x9d IEEE ISCAS, pp. 97-100, 1999. In use, the equalizer is included in a receiver that is coupled to receive a signal that has propagated over a cable. The received signal (labeled xe2x80x9cinxe2x80x9d in FIG. 1) is filtered in low-frequency-gain path 1 (sometimes referred to as low-frequency filter 1) and high-frequency-boosting path 2 (sometimes referred to as high-frequency filter 2, and which includes a high-pass filter in series with an amplifier having adjustable gain). In some implementations, filter 1 applies gain (which can be unity gain) to the received signal. In other implementations (in which filter 1 includes a low-pass filter), filter 1 applies gain to low-frequency components of the received signal but not to high-frequency components thereof. The outputs of filters 1 and 2 are combined in adder 4, the output of adder 4 enters regulating comparator 6, and the output of comparator 6 is limited to have a predefined maximum amplitude. To decide the amount of high-frequency-boost applied by filter 2, a servo loop compares the slope of the comparator""s input signal against that of the comparator""s output (the input to comparator 6 is filtered in high-pass filter 5, the output of filter 5 is asserted to rectifier 7, the output of comparator 6 is filtered in high-pass filter 8, the output of filter 8 is asserted to rectifier 9, and error amplifier 10 asserts to the adjustable-gain element of high-frequency filter 2 an equalization control signal indicative of the difference between the outputs of rectifiers 9 and 7). The control signal asserted by error amplifier 10 tries to make the slope of the comparator""s input and output signals the same. The difference between the slope of the comparator""s input and output signals is measured by comparing high-pass-filtered (and rectified) versions of the comparator""s input and output signals using error amplifier 10 as shown in FIG. 1.
The signals asserted to the inputs of error amplifier 10 are related to the amplitudes of the input and output signals of comparator 6. The amplitude of the output of comparator 6 is determined by the amplitude of the input to, and the regulation performed by, comparator 6. The amplitude of the input to comparator 6 is determined by the combination of the transmitter""s driving voltage amplitude, the cable""s flat loss and frequency-dependent low-frequency attenuation, and the gain of the filter""s low-frequency path 1. If the signal amplitudes at the input and output of comparator 6 are different, the high frequency component of the difference between the signals at the input and output of comparator 6 is not a function only of channel attenuation and the high-frequency boosting applied by high-frequency filter 2. Rather, the high frequency component of the difference between the signals at the input and output of comparator 6 is affected by comparator 6""s regulation level and the overall low-frequency gain applied by filters 1 and 2. If the low frequency gain deviates from the ideal one, the result of high-frequency-boosting adaptation will be degraded. To eliminate this source of degradation, a joint adaptation algorithm is implemented in accordance with the invention, to adapt not only high frequency content but also low frequency content.
In a class of embodiments, the invention is an adaptive equalization circuit that implements a joint adaptation algorithm. The equalization circuit includes a filter having a low-frequency-gain path (sometimes referred to as a low-frequency filter) and a high-frequency-boosting path (sometimes referred to as a high-frequency filter). The high-frequency filter typically includes a high-pass filter in series with an amplifier having adjustable gain. A high-frequency-boosting tuning loop controls the adjustable gain applied by the high-frequency filter. A low-frequency-gain tuning loop controls the adjustable gain applied by the low-frequency filter.
Preferably, the high-frequency-boosting tuning loop includes the high-frequency filter, a comparator (e.g., a regulating comparator), a first high-pass filter coupled to the input of the comparator, a second high-pass filter coupled to the output of the comparator, a first rectifier coupled to the output of the first high-pass filter, a second rectifier coupled to the output of the second high-pass filter, and an error amplifier for generating an equalization control signal in response to the outputs of the first rectifier and the second rectifier. Preferably, the low-frequency-boosting tuning loop includes the low-frequency filter, the comparator, a first low-pass filter coupled to the input of the comparator, a second low-pass filter coupled to the output of the comparator, a third rectifier coupled to the output of the first low-pass filter, a fourth rectifier coupled to the output of the second low-pass filter, and a second error amplifier for generating an second equalization control signal in response to the outputs of the third rectifier and the fourth rectifier.
Other aspects of the invention are a receiver that includes any embodiment of the inventive adaptive equalization circuit, and a joint adaptation equalization method.